In the drilling of wells, such as oil or gas wells, cores are taken of the earth formation through which the wells are drilled and various characteristics of the cores, or core samples, are determined for the purpose of establishing different fluids in the formation, estimating the quantity of each fluid in the formation, the ease of flow through the formation, etc. Such core samples are also taken from producing reservoirs and characteristics of the core samples are determined for the purpose of estimating particular fluid quantities, predicting production rates, etc. Among the characteristics of core samples commonly determined is the formation resistivity factor involving the measurement of the electrical resistivity of the core samples. Such resistivity factor is set forth in "The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics", Trans. AIME, Vol. 46, pp. 54-62, 1942, by G. E. Archie.
Standard practice is to measure porous rock resistivities for a number of partial water saturations and to plot a resistivity index I, determined from the ratio of the measured resistivity of the rock at a partial water saturation to the measured resistivity at complete water saturation, against the water saturation at which the resistivity measurements were made. From this plot the rate of change of rock resistivity with varying water saturation, defined by Archie as a saturation exponent, is determined. However, such a plot assumes that all rock pores are desaturated equally and all resistivities for partial water saturation are measured under an equilibrium distribution of a conducting and a nonconducting fluid, e.g., water (brine) and oil. These fluids must be immiscible throughout the rock sample. If an equilibrium distribution of fluids is not reached throughout the rock sample at each partial water saturation, then the measured resistivities will not be correct and any saturation exponent determined from such resistivities will not be characteristic of the rock.
It is therefore a specific objective of the present invention to provide apparatus for measuring rock resistivities under varying water saturations which can be positively identified as being in a state of fluid distribution equilibrium at the time the resistivity measurements are made.